Dr. Bob on Comparison and Performance: EIA 364D Test Group 2
This test group is the longest of the groups in the EIA 364D generic test sequence. A copy of the test group is attached for your convenience. The entire test sequence can also be found in the first article in this series.
Test group 2 is a hybrid test group, in that it includes mechanical and corrosion conditioning/exposures and mechanical and electrical measurements.
The test begins with two measurements, mating/unmating force (M/UF, TP 13), followed by electrical resistance (generally low-level contact resistance, LLCR, TP 23) to establish a base line for these two parameters.
The conditioning/exposure begins with durability cycling (TP 09). TP 09 defines cycling procedures but does not specify the number of durability cycles. That decision is up to the requestor of the test. Various criteria for the number of mating cycles can be considered, including:
The rated durability of the connector according to the relevant specification.
Some fraction of the rated durability based on an expected value of the durability cycles in the application under evaluation.
A “standard” or benchmark value obtained from a specification for the generic or market application in which the connector will be used.
In my opinion, the selection of the number of durability cycles based on these considerations distinguishes between whether durability cycling is a conditioning or exposure. Decisions according to “a” or “b” above are exposures, because they relate to the specification or application under evaluation. A decision according to “c” is a conditioning step because it has a generic character and will be useful only in a comparative sense to distinguish among different connectors intended for the same markets. Similar comments would, of course, apply to other exposures, such as thermal or mechanical shock, among others.
The durability cycling is followed by repeating the M/UF and LLCR measurements. The M/UF measurement is relevant in that it could detect any significant effects of connector mating on the normal force of the contact beams. I suggest, however, that the LLCR measurement will generally not be relevant. The reasoning behind this statement is that durability cycling can produce wear and damage to the contact finish that could affect the contact resistance, but the effects of any such degradation would not be apparent until the damage to the finish had been exposed to a corrosive environment which caused films or contaminants to form in or around the contact interface. Multiple mating cycles will generally produce a clean contact interface in the absence of corrosion exposures, and would not result in a change in contact interface resistance.
The next conditioning/exposure is thermal shock (TS, TP 32). Again, the procedure is defined in TP 32, but the details of the conditioning/exposure—temperature range, temperature transition, dwell times, and number of cycles—are not. The same general comments as made previously with respect to durability cycling, would apply to this conditioning/exposure.
The next step in the sequence, LLCR, is optional. The comments on the relevance of an LLCR measurement after durability apply in this case as well, given that thermal shock is an alternative method of disturbing the contact interface.
The next step is humidity conditioning/exposure (TP 31). TP31 includes both steady state and cyclic humidity options, and lists options for test conditions and exposures. Cyclic humidity is generally preferred over steady state, as it is more representative of application conditions. As before, selection from the available options, or definition of alternative conditions, rests with the requester of the test program.
Humidity conditioning provides a potential driving force for the corrosion/contamination in and around the contact interface mentioned previously and, thus, the final step, electrical resistance, is relevant. Humidity exposures are more appropriate for tin-finished connectors than to noble metal—for example, gold-finished connectors. For noble metal finishes, a mixed flowing gas exposure is preferred.
If this test sequence is used in the comparative sense described previously, the sequence of thermal shock followed by humidity is relevant. If, however, the intention is to assess performance in a particular application, I suggest that the reverse order, durability cycling – humidity – thermal shock, would be more relevant. As mentioned, thermal shock is another driving force for “disturbance” of the contact interface, and, in itself, will not generally lead to contact resistance degradation. In the reverse order, the durability cycling provides a driving force for assessing the mechanical stability of the contact interface, or for interface degradation. The humidity exposure provides an opportunity for corrosion/contamination in and around the potentially “degraded” contact interface. And, finally, thermal shock provides another mechanical stability evaluation of a potentially degraded contact interface, with resultant increases in contact resistance. In other words, the test sequence should be derived from a consideration of how the various conditioning or exposure steps will impact the degradation of connector performance. We will revisit this issue again in further articles in this series.
Consideration of Test Group 2 of the generic test sequence of EIA 364D has highlighted some of the important issues relating to the purpose of testing. Two purposes have been identified in this discussion, comparative testing and performance assessment. In a comparative assessment, a standardized sequence using standard conditioning, that is, for example, a prescribed number of thermal shock cycles at given values of temperature range and dwell times, is relevant. If the purpose of testing is performance assessment, the conditioning—or in this case what I call exposure—parameters must be tailored to represent the conditions expected in the specific application under consideration. While the selection of the exposure severity and duration parameters is “arbitrary,” the rationale for those parameters must be grounded in a considered assessment of the application conditions and expected life of the connector in the field. If reliability assessment is the objective of the test, additional considerations concerning the relationship of the test exposures to field conditions, both in simulating the degradation mechanisms of interest, and in the “acceleration factor” between test and field exposures, pose new and even more challenging issues.
In the next article in this series, Max Peel, of Contech Research, will provide his practical perspective on Test Group 2.
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